Lung cancer is the most common cause of cancer-related deaths in the United States. The cure rate for patients with advanced lung cancer remains low and has not changed significantly for the last 30 years. A better understanding of the signaling pathways important in driving and maintaining the malignant state is important for continued progress in the treatment of patients with lung cancer. The Notch family of receptors has been demonstrated to be important both in cell fate determination and tumorigenesis. Despite the increasing role of Notch pathway in human cancers, very little was known about the role of Notch3 and its effectors in lung cancers. We were the first to link dysregulation of the Notch3 pathway to human lung cancer. We demonstrated that NotchS is highly expressed in 40% of all resected lung cancers and that, in the developing lung, constitutive activation of NotchS results in inhibition of terminal differentiation of pneumocytes. Furthermore, we have shown that inhibiting this pathway in human lung tumors results in the loss of the malignant phenotype in vitro and in vivo using xenograft models. This antitumor effect is enhanced in the presence of low serum and in combination with an EGFr tyrosine kinase inhibitor. Taken together, our data support an important role for NotchS receptor and its interaction with the EGF and ras pathways in lung cancer. In addition, we now have strong evidence from both in vitro and in vivo studies that Notch signaling can be effectively blocked by inhibiting the required receptor processing with a y-secretase inhibitor from Merck Inc. &Co., currently in Phase I human clinical trials. In this proposal, we will further develop preclinical and clinical models to optimize these strategies using Y-secretase inhibitor for human clinical testing. Furthermore, we also propose to develop biologically active, recombinant antibody targeting NotchS. Both these approaches represent novel, therapeutic strategies in the treatment of patients with lung cancer.